1. Field of the Invention
The present invention relates generally to direct sequence spread spectrum (code division multiple access) receivers. This invention is particularly concerned with a sync acquisition and tracking technique for direct sequence spread spectrum receivers of cellular mobile communications systems in which transmitted signals are severely affected by Rayleigh fading and multipath fading.
2. Description of the Related Art
Commercial interest in direct sequence spread spectrum (code division multiple access) communication systems has recently risen due to their potential ability to provide service to more users than is offered by other multiple access techniques. In the cell-site station of the DS/CDMA system, a data symbol is spread by multiplying it with orthogonal pseudo-random number (PN) sequences assigned to the cell site as well as with orthogonal Walsh codes assigned to the channel over which the spread signal is transmitted. In order to enable the mobile station to provide sync acquisition and tracking operation, a pilot signal is superimposed on the data symbol sequence. At the mobile station, a sliding correlation technique is used to shift the phase timing of a local PN sequence by a predetermined amount each time a correlation is taken between the received and local sequences and determine the correct phase timing for the local sequence when the correlation exceeds some critical value. Such phase shifting is performed at predetermined "window" intervals. Once synchronization is established, the phase difference is monitored and maintained to within one half of the chip interval. During transmission, the signal undergoes reflections from various land structures, producing a complex pattern of standing waves due to mutual interference. As a result, the propagation path of the signal exhibits a field intensity distribution which is approximated by the Rayleigh distribution. Thus, the signal experiences a phenomenon called "Rayleigh fading" and the envelope of the signal at the mobile station as well as its phase violently fluctuate.
Under such unfavorable conditions, the sliding correlation technique is not ideal to achieve quick synchronization in response to the rapidly varying signal levels. In addition, there is often a need to update the mobile's phase timing due to the arrival of a strong reflection or a signal of significant level from an adjacent cell site. Under such circumstances, the prior art system sets the correlation circuitry to the new phase timing immediately in response to the arrival of a new strong signal without taking the old symbols, which may be left in the correlation circuitry, into account. The use of the old symbols for the new phase timing results in timing discontinuity and produces over-correlation or under-correlation.